This invention relates to mercury vapor fluorescent lamps and particularly to a method for maintaining the mercury pressure within the lamp at an optimum value by monitoring and controlling the lamp output voltage.
In a mercury fluorescent lamp, an electrical discharge is generated in mercury vapor at low pressure and typically mixed with argon gas. The light output from the lamp depends, amoung other variables, on the mercury vapor pressure inside the lamp tube. The primary radiation from the mercury is at 2537 Angstroms and arises from the transition between the lowest nonmetastable excited state and the ground state. This ultraviolet radiation at 2537 Angstroms excites a phosphor which is coated inside the tube walls. The excited phosphor thereupon emits radiation at some wavelength, in the visible spectrum, characteristic of the phosphor.
It is known in the prior art that the optimum mercury pressure for maximum light output of a fluorescent lamp in alternating current operation is approximately 7 mtorr (independent of current) which corresponds to a mercury cold spot temperature of approximately 42.degree. C. At this temperature and pressure, the light output increases monotonically with the current. At cold spot temperatures higher or lower than the optimum, light output falls off.
It is therefore desirable to maintain the mercury pressure at the optimum at any lamp current and at any ambient temperature. Prior art techniques for accomplishing this function required a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit provided closed loop control of a temperature-regulating device to maintain the optimum mercury pressure. These methods, although providing a closed loop control of the cold spot temperature, must rely on a consistent relationship of cold spot temperature to light output which may not exist under all conditions.
The present invention is directed to a novel method for maintaining optimum mercury pressure which does not require the use of cold spot temperature measuring devices. As will be demonstrated in the suceeding descriptive portion of the specification, if lamp current is kept constant, the lamp arc voltage (voltage drop across the lamp) is a function of the mercury cold spot temperature. This voltage perks at approximately the same cold spot temperature as does the light output. According to one aspect of the invention, the lamp voltage is continually monitored by a circuit which is adapted to feed back a signal to a cold spot temperature-regulating device under certain condition. The circuit responds to any reduction in the voltage by reversing the operating mode of the temperature-regulating device. Thus, if the device has been off it is turned on and if on, it is turned off. Either action has the effect of restoring the output voltage to its peak level, and hence restoring the optimum mercury pressure.
A prime advantage of the method of the invention is that once the distribution and feedback circuits are designed with the appropriate algorithm, the system does not require any absolute calibration; that is, the peak voltage for a particular lamp does not need to be determined. Further, the feedback circuit is extremely fast relative to the prior art feedback loop which required a longer response time due to the thermal mass of the mercury pool heat sink, the glass envelope and the temperature sensitive device.
The present invention is therefore directed to a monitoring and control mechanism for optimizing the light output of a fluorescent lamp containing an excess of mercury at a cold spot therein, said mechanism comprising;
a power supply for applying operating current to said lamp,
temperature control means adapted to operate in a first mode whereby temperature at said cold spot is increasing and in a second mode whereby temperature at said cold spot is decreasing, and
a monitoring means for detecting a drop in the arc voltage of said lamp, said monitoring means adapted to transmit a signal to said temperature control means changing the instant mode of operation.